This invention relates to controlling the intensity of the electric field in an electrostatic separation process and in particular to a voltage control system for controlling the intensity of the electric field when removing water from an emulsion of oil and water in an electrostatic oil dehydrater.
Separation processes utilizing high voltage electric fields include solid-solid separation in the benefaction of ores, solid or liquid separation from a dielectric liquid, and the use of electrostatic precipitators to remove solid or liquid particles from exhaust gases and air conditioning systems. The electrostatic field utilized in these separation processes is generated by applying an alternating current voltage, a direct current voltage, or some combination thereof, to a grid within the process flow.
When the liquid-liquid separation removes water from an oil-water emulsion in an electrostatic dehydrater, the oil acts as a dielectric and water droplets are coalesced. Coalescence occurs when small water droplets collide and unite to form larger water droplets. Water droplets are coalesced by establishing an electric field between grid elements and passing the oil-water emulsion through the electric field. Since water is slightly polar, it will move toward the oppositely charged grid element, coalesce and gravitate to the bottom of the dehydrater where the water is removed.
Small water droplets have a low settling velocity and therefor gravitate slowly or are carried along with the through flow. It is desirable to cause small water droplets to coalesce to form larger water droplets since the larger water droplets gravitate more readily. Small water droplets, however, are more difficult to move through oil and therefore require more power to coalesce. Specifically, to coalesce small water droplets, an intense electric field such as is achieved when a high voltage is applied to the grid elements is required. A problem arises in maintaining a high voltage applied to the grid elements in that during the coalescence process eventually the water droplets become sufficiently large that the high voltage operates detrimentally to the coalescence process by shearing the larger water droplets into smaller water droplets.
U.S. Pat. No. 4,204,934 addresses this problem. In FIG. 2 an arrangement is disclosed wherein a larger voltage is applied to grid element 22 than is applied to grid element 23 creating a higher electric field intensity between grid element 22 and ground element 21 than between grid element 23 and ground element 21. The through flow first passes through the higher intensity electric field to coalesce small water droplets then through the lower intensity electric field to continue the coalescence process without shearing the larger coalesced droplets. FIG. 3 discloses an alternate arrangement wherein the same voltage is applied to grid elements at different physical distances from ground thereby creating a higher electric field intensity between the closer spaced grid element and ground while establishing a lower electric field intensity between the more distant spaced grid element and ground. Again the through flow first passes through the higher intensity electric field to coalesce small droplets then through the lower intensity electric field to continue the coalescence process without shearing the larger coalesced droplets. FIG. 4 discloses yet another arrangement wherein a voltage is applied to a single grid element with non-uniform physical spacing between the grid element and electrical ground. The through flow first passes through a low intensity electric field where the grid element and electric ground are spaced apart, then through a high intensity electric field where the grid element and electrical ground converge toward each other and finally through a decreasing intensity electric field where the grid element and electrical ground diverge.
What is needed is a method and apparatus for varying the electric field intensity between grid elements in an electrostatic separation process that would subject through flow to a high electric field intensity to coalesce small water droplets followed by a lower electric field intensity to reduce the shear forces to which the coalesced water droplets are exposed so that coalescence of larger water droplets continues without rupturing. Such a method and apparatus would be more flexible than the physical arrangement of grids and could be adjusted for the specific application to enhance the separation process.